1. Field of the Invention
The present invention relates to an improvement in a programed exposure control device.
2. Description of the Prior Art
Such a programed exposure control device for lens shutter has been known as having a shutter which acts also as an aperture in such a way that, when the object is bright, the shutter blades are closed before they are moved to full opening position. For the cameras of the type including other types than the lens shutter, as well as the lens shutter, there are several types of programed exposure control device. For instance, a first type device is arranged such that the aperture opening and the shutter speed are determined simultaneously in acccordance with the brightness of the object. Also, a second type device is arranged such that the aperture opening is selected in accordance with the brightness of the object and then the shutter speed is determined in accordance with the selected aperture opening and the brightness of the object.
Excepting the programed lens shutter, in the above-mentioned exposure control device of the first type, the aperture opening and the shutter speed are determined simultaneously with each other. It is, however, extremely difficult to set the aperture opening and the shutter speed precisely in accordance with the program. This not only complicates the construction of the device but also poses a problem that a considerably long time is required for the setting of the aperture opening and shutter speed. In the device of the second type, at first the aperture opening is set and then the shutter speed is determined on the basis of the aperture opening and the brightness of the object, so that the setting of the aperture opening does not necessitate such a high precision and, therefore, it is possible to promptly determine the aperture opening. However, since the shutter speed is determined after the setting of the aperture opening, the whole process of determination of the exposure condition requires a considerably long time. In addition, a considerably complicated mechanism is required for the setting of the aperture opening in accordance with the brightness of the object.
In order to eliminate the above-described problems in the programed exposure control device not limited to the lens shutter, the present inventor has already proposed a programed exposure control device in which the object brightness information, with or without correction by film sensitivity or other exposure information, is suitably ranked to permit determination of the aperture opening fixed for each rank, and the shutter speed is determined in accordance with the rank and the object brightness information.
In this exposure control device, the aperture opening is determined in a stepped manner according to the rank of the object brightness information, so that the aperture setting device can have a simplified and strong construction while achieving a prompt setting of the aperture opening. The aperture opening is set in a stepped manner in accordance with the rank, so that, once the ranking of the object brightness information is made, it is possible to determine the aperture opening and the shutter speed simultaneously. In consequence, the exposure condition is set in quite a short period of time to avoid loss of the chance of shooting.
When the above-described invention is applied to a changeable-lens type camera, it is necessary to correct the object brightness information in accordance with the F value. When the brightness information thus corrected is ranked, it is considerable that the aperture opening corresponding to the rank is greater than the maximum aperture opening, i.e. F value, of the new lens. In this case, it is not possible to set the aperture in conformity with the ranking of the brightness information. This problem can be overcome by changing the ranking in such a manner that the aperture opening takes the same value as the F value of full opening of the new lens or slightly smaller than the F value. This solution, however, is inconvenient in that the number of steps of the ranks available is reduced and, in addition, the determination of the shutter speed is made only at extremely late part in the ranking. This may cause the photograph to be blurred by a slight movement of hand during taking the picture.
In the TTL open light measuring program type exposure control device for the conventional exchangeable lens type camera, the TTL light information is affected by the open F value of the lens. The TTL light information, therefore, is corrected by means of the open F value information. In consequence, the exposure control device is complicated and it becomes necessary to provide the lens unit with a member for the introduction of the open F value. This may cause some failures in the handling of the camera.